Λp femtoscopy in collisions of Ar + KCl at 1.76 A GeV with HADES

Results on Λp femtoscopy are reported at the lowest energy so far. At a beam energy of 1.76.A GeV, the reaction Ar + KCl was studied with HADES at SIS18/GSI. A high-statistics and high-purity Λ sample was collected, allowing for the investigation of Λp correlations at small relative momenta. The experimental correlation function is compared to corresponding model calculations allowing the determination of the space-time extent of the Λp emission source. The Λp radius is found significantly smaller than that for Au + Au/Pb + Pb collisions in the AGS, SPS and RHIC energy domains, but larger than that for electroproduction from He. Taking into account all available data, we find the Λp source radius to increase almost linearly with the number of participants to the power of one-third.     

Production of charged pions, kaons and antikaons in relativistic C + C and C + Au collisions

Production cross-sections of charged pions, kaons and antikaons have been measured in C+C and C+Au collisions at beam energies of 1.0 and 1.8 AGeV for different polar emission angles. The kaon and antikaon energy spectra can be described by Boltzmann distributions whereas the pion spectra exhibit an additional enhancement at low energies. The pion multiplicity per participating nucleon M(π⁺)/<A _part> is a factor of about 3 smaller in C+Au than in C+C collisions at 1.0 AGeV whereas it differs only little for the C and the Au target at a beam energy of 1.8 AGeV. The K⁺ multiplicities per participating nucleon M(K⁺)/ <A _part> are independent of the target size at 1 AGeV and at 1.8 AGeV. The K^- multiplicity per participating nucleon M(K^-)/ <A _part> is reduced by a factor of about 2 in C+Au as compared to C+C collisions at 1.8 AGeV. This effect might be caused by the absorption of antikaons in the heavy target nucleus. Transport model calculations underestimate the K^-/K⁺ ratio for C+C collisions at 1.8 AGeV by a factor of about 4 if in-medium modifications of K-mesons are neglected. Received: 10 December 1999 / Accepted: 14 November 2000     

Protons flow in Pb+Pb collisions at 40 <Emphasis Type="Italic">A</Emphasis> GeV energy

For Pb+Pb collisions at 40 A GeV energy, we calculate the side-ward and elliptic differential flow of protons in the microscopic relativistic transport simulation model. We compare our results with the recent data from the NA49 Collaboration as a function of transverse momenta, rapidity and centrality. We find that the side-ward and elliptic flow agree reasonably well with the experimental data with and without momentum-dependent potentials in the simulation model.     

Two-particle azimuthal correlations of high-pT charged hadrons at the CERN SPS

Two-particle azimuthal correlations of high-p _T hadrons can serve as a probe of interactions of partons with the dense medium produced in high-energy heavy-ion collisions. First NA49 results on such correlations are presented for central and mid-central Pb+Pb collisions at 158A GeV beam energy, for different centrality bins and charge combinations of trigger and associate particles. These results feature a flattened away-side peak in the most central collisions, which is consistent with expectations of the medium-interaction scenario. A comparison with CERES Pb+Au results at the same energy, as well as with PHENIX Au+Au results at the top RHIC energy, is provided.     

Search for Hawking radiation in heavy ion collisions

The creation of “white holes” that decay by Hawking radiation has been proposed as one way to achieve the very early thermalization observed in heavy ion collisions at RHIC. The charartistic temperature of the radiations depends only on the ratio of the baryon number to the transverse energy. The yields of pions, kaons, protons and antiprotons measured by BRAHMS in central Au+Au collisions can be described within a thermal model where T drops with rapidity, and beam energy. We find that the chemical freeze-out temperature drops as the ratio of baryon number to energy increases but much more rapidly than predicted by the model.     

STAR measurement of system size and incident energy dependence of pt correlations at RHIC

We present the results on measurement of event-by-event p _t fluctuations and correlations for Cu+Cu collisions at ?{s_NN }\sqrt {s_{NN} } = 62.4 and 200 GeV from STAR experiment at RHIC. These results are compared with those from Au+Au collisions at ?{s_NN }\sqrt {s_{NN} } = 62.4 and 200 GeV to study the system size dependence. We find that the dynamical p _t fluctuations are finite and decrease with increasing collision centrality. The p _t correlations are studied as a function of collision centrality and are observed to decrease as we go from peripheral to central collisions. The square root of p _t correlations scaled by mean p _t are observed to be independent of beam energy as well as colliding ion size.     

Longitudinal flow of protons from (2-8)A GeV central Au+Au collisions

Rapidity distributions of protons from central 197Au+197Au collisions measured by the E895 Collaboration in the energy range from (2-8)A GeV at the Brookhaven AGS are presented. Longitudinal flow parameters derived using a thermal model including collective longitudinal expansion are extracted from these distributions. The results show an approximately linear increase in the longitudinal flow velocity, (L), as a function of the logarithm of beam energy.     

Comparison of source images for protons, pi-'s, and lambda's in 6A GeV Au+Au collisions

Source images are extracted from two-particle correlations constructed from strange and nonstrange hadrons produced in 6A GeV Au+Au collisions. Very different source images result from pp vs p Lambda vs pi(-)pi(-) correlations. Scaling by transverse mass can describe the apparent source size ratio for p/pi(-) but not for Lambda/pi(-) or Lambda/p. These observations suggest important differences in the space-time emission histories for protons, pions, and neutral strange baryons produced in the same events.     

Evidence for a Soft Nuclear Equation-of-State from Kaon Production in Heavy-Ion Collisions

The production of pions and kaons has been measured in 197Au+197Au collisions at beam energies from 0.6 to 1.5A GeV with the kaon spectrometer at SIS/GSI. The K+ meson multiplicity per nucleon is enhanced in Au+Au collisions by factors up to 6 relative to C+C reactions, whereas the corresponding pion ratio is reduced. The ratio of the K+ meson excitation functions for Au+Au and C+C collisions increases with decreasing beam energy. This behavior is expected for a soft nuclear equation-of-state.     

Effects of the HMT on Nucleon Collective Flows within BUU Transport Model

Within the framework of a semiclassical Boltzmann-Uehling-Uhlenbeck(BUU) transport model,the high momentum tail(HMT) effects of nucleon momentum distribution in the nucleus on the nucleon collective Bows are studied in semicentral Au+Au collisions.The HMT due to the isospin-dependent short-range correlations causes a smaller value of the collective Bows.We Bnd that the HMT effects on the nucleon collective Bows are remarkable at beam energy of 300 MeV/nucleon and become weak as the incident beam energy increases.The results indicate that for the collective Bow studies at intermediate energies,the HMT of nucleon momentum distribution in nucleus should be taken into account in transport models.     

Search for the onset of baryon anomaly at RHIC-PHENIX

The baryon production mechanism at the intermediate p_T (2–5 GeV/c) at RHIC is still not well understood. The beam energy scan data in Cu+Cu and Au+Au systems at RHIC may provide us a further insight on the origin of the baryon anomaly and its evolution as a function of . In 2005 RHIC physics program, the PHENIX experiment accumulated the first intensive low beam energy data in Cu+Cu collisions. We present the preliminary results of identified charged hadron spectra in Cu+Cu at and 62.4 GeV using the PHENIX detector. The centrality and beam energy dependences of (anti)proton to pion ratios and the nuclear modification factors for charged pions and (anti)protons are presented. PACS 25.75.Dw     

Subthreshold K<Superscript>-</Superscript>-meson production in proton-nucleus reactions revisited

We study the inclusive K^--meson production in proton-nucleus collisions in the subthreshold energy regime in the framework of a spectral function approach for incoherent primary proton-nucleon and secondary pion-nucleon production processes. Our approach takes properly into account the effect of the nuclear mean-field potentials on these processes, as well as the final-state interaction (FSI) among the outgoing nucleons participating in the one-step antikaon creation process. A detailed comparison of the model calculations of the K^- differential cross-sections for the reactions p + ⁹Be, p + ⁶³Cu, and p + ¹⁹⁷Au at subthreshold energies with the currently available experimental data obtained recently at the ITEP proton synchrotron and at SIS/GSI is given. We find that our calculation, which includes both the nuclear density-dependent mean-field potentials and the elementary NN-FSI effects on the K^- production from the direct mechanism, is able to reproduce the energy dependences of the invariant differential cross-sections for the “hard” antikaon creation in p- ⁹Be and p- ⁶³Cu collisions. This result contradicts previous estimates which used only density-dependent mean fields in calculating the K^- yield from this mechanism. We further show that the NN-FSI effects play a minor role in describing the data on the spectrum of the relatively soft K^--mesons from p- ¹⁹⁷Au interactions at incident energy of 2.5GeV. We find that the relative strength of the proton- and pion-induced reaction channels in the subthreshold energy regime is governed by the kinematics of the experiment under consideration. We also explore the influence of the antikaon mean-field potential on the K^- yield at low antikaon momenta. Received: 26 July 2001 / Accepted: 23 December 2002 / Published online: 8 April 2003     

Sideward flow in Au+Au collisions between 2A and 8A GeV

Using the large acceptance Time Projection Chamber of experiment E895 at Brookhaven, measurements of collective sideward flow in Au+Au collisions at beam energies of 2A, 4A, 6A, and 8A GeV are presented in the form of in-plane transverse momentum and the first Fourier coefficient of azimuthal anisotropy v(1). These measurements indicate a smooth variation of sideward flow as a function of beam energy. The data are compared with four nuclear transport models which have an orientation towards this energy range. All four exhibit some qualitative trends similar to those found in the data, although none show a consistent pattern of agreement within experimental uncertainties.     

Excitation functions of fusion reactions and neutron transfer in the interaction of 6He with 197Au and 206Pb

Excitation functions for evaporation residues in the reactions ¹⁹⁷Au(⁶He, xn)^203-xnTl, x = 2-7, and ²⁰⁶Pb(⁶He, 2n)²¹⁰Po, as well as for neutron transfer reactions for the production of ¹⁹⁶Au and ¹⁹⁸Au in the interaction of ⁶He with ¹⁹⁷Au were measured. The ⁶He beam was obtained from the accelerator complex for radioactive beams DRIBs (JINR). The maximum energy of the beam was about 10AMeV and the intensity reached 2×10⁷pps. The stacked-foil activation technique was used directly in the beam extracted from the cyclotron or in the focal plane of the magnetic spectrometer MSP-144. The identification of the reaction products was done by their radioactive γ- or α-decay. The fusion reaction with the evaporation of two neutrons was characterized by an increase in the cross-section compared to statistical model calculations. The analysis of the data in the framework of the statistical model for the decay of excited nuclei, which took into account the sequential fusion of ⁶He has shown good agreement between the experimental and the calculated values of the cross-sections in the case of sub-Coulomb-barrier fusion in the ²⁰⁶Pb + ⁶He reaction. An unusually large cross-section was observed below the Coulomb barrier for the production of ¹⁹⁸Au in the interaction of ⁶He with ¹⁹⁷Au. Possible mechanisms of formation and decay of transfer reaction products are discussed.     

Production of secondaries in high-energy <Emphasis Type="Italic">d</Emphasis>+Au collisions

In the framework of the quark–gluon string model we calculate the inclusive spectra of secondaries produced in d+Au collisions at intermediate (CERN SPS) and at much higher (RHIC) energies. The results of numerical calculations at intermediate energies are in reasonable agreement with the data. At RHIC energies numerically large inelastic screening corrections (percolation effects) should be accounted for in the calculations. We extract these effects from the existing experimental data of RHIC on minimum-bias and central d+Au collisions. The predictions for p+Au interactions at LHC energy are also given.     

Testing the mechanism of QGP-Induced energy loss

We present an analytic model of jet quenching, based on the (D)GLV energy loss formalism, to describe the system size dependence of QGP-induced parton absorption in relativistic heavy ion collisions. Numerical simulations of the transverse momentum dependence of jet quenching are given for central Au+Au and Cu+Cu reactions. Low p _Tdijet correlations are shown to be sensitive to the reappearance of the lost energy as soft hadrons. At high p _Twe find that the attenuation of dihadrons is similar to that of single inclusive particles. Comparison to recent data from PHENIX and STAR is given as a test of the jet quenching theory.     

Dynamic model of mass-angular correlations of quasi-fission fragments

A dynamic model of the mass-angular correlations of quasi-fission fragments was developed. The projection of the angular momentum onto the symmetry axis of the dinuclear system and its rotation angle are treated as dynamic variables in the model. The model was tested in the analysis of the experimental data for the ⁶⁴Ni + ¹⁹⁸Au reaction at E _lab = 383 and 418 MeV. Information on lifetimes of the dinuclear system was obtained.     

Proton emission in α-induced reactions at 43 MeVnucleon

Inclusive proton spectra and proton-proton correlations have been measured from α + ⁵⁸Ni and α + ¹⁹⁷Au reactions at energies E_α = 100 MeV and 172 MeV. The inclusive spectra are compared to results of a simple model assuming local equilibration of the energy transferred to the target nucleons. This model describes the energy and angular dependence sufficiently well at angles ? 40°. Due to finite number effects, it also explains the occurrence of differences between in-plane to out-of-plane pp correlations. In addition, the measured coincidences indicate contributions from quasifree pp scattering.     

Oscillating HBT radii and the time evolution of the source—√sNN = 200 GeV Au + Au data analyzed with azimuthally sensitive Buda-Lund hydro model

Identified particle spectra of pions, kaons and (anti)protons, and elliptic flow and azimuthal dependence of Bose-Einstein or HBT correlations of identified pions in s _NN = 200 GeV Au + Au collisions is analyzed simultaneously using an ellipsoidally symmetric generalization of the Buda-Lund hydrodynamical model. The transverse flow is found to be faster in the reaction plane than out of plane, which results in a reaction zone that gets slightly more elongated in-plane than out of plane.     

Emission order of light particles and light fragments from hot nuclei followed by binary fission

Emission orders of light charged particles and Li fragments from highly excited fissioning nuclei in the reaction of ⁴⁰Ar+¹⁹⁷Au at E _beam /A=25 MeV have been studied by measuring difference velocity distributions of two correlated particles at small relative angles in coincidence with two fission fragments. By comparing the data with three-body trajectory calculations, we found that high velocity deuterons are emitted prior to high velocity Li fragments but low velocity deuterons are emitted after low velocity Li fragments. On the other hand, no preferential emission was observed among light particles, such as protons and deuterons. Furthermore, the emission orders are found to depend only weakly on the mass asymmetry of fission fragments. Received: 3 June 1997 / Revised version: 23 July 1997